[isomorphisms]

Stacks on PR #67, “Separate hyperplane fit partitions from semantic labels.”

The affine-plane mechanics were originally coupled to the arXiv body experiment's representation assertion title_or_url_used=false. Private Takeout needs to reuse the plane boundary without pretending a YouTube title+channel representation is body text.

This change:

• keeps pensieve-body-only as the default input contract and preserves its no-title/no-URL assertion
• adds an explicit declared-text contract for external corpora
• requires declared external input manifests to name a nonempty representation kind and the exact model-text fields
• keeps input SHA binding and embedding-provenance binding unchanged
• records the representation contract in the probe report
• adds positive and known-bad representation-contract fixtures
• runs hyperplane CI on 0.2/takeout-* experiment branches so stacked exact-head work gets evidence

This does not make title text equivalent to body text and does not change the SVM, threshold, vote, partition, or evaluation mechanics.